Gonadal steroid effects on anatomy and behavior: This project will investigate the effects of gonadal steroids on the on the expression of courtship display structures and behavior in African mormyrid fish. It is based on earlier studies that demonstrated in females some androgen- induced changes (behavior) were temporary and others (bone, muscle) long-lasting; androgen-treated males may turn into "supermales" with exaggerated structural traits (bone). -One can mimic natural processes of maturation in these fish through sex steroid manipulation, and study the mechanisms that direct structural and behavioral transformations. Mormyrid cellular bone resembles mammalian bone. Knowledge about bone metabolism derived from the mormyrid preparation can render it a valuable research tool. -Androgen treated juvenile and adult females develop massive anal-fin ray bone expansion, characteristic of males. This expansion apparently provides the necessary substrate for musculature which in turn executes the anal fin reflex (forming a common spawning pouch). -Integrating organismic, cellular, and molecular levels of analysis these studies determine (1) the extent of gonadal steroid- induced transformations of bone (using radiography) and behavior (anal- fin reflex, electric organ discharge), (2) the anatomy of (a) the anal fin and its motoneuronal control (b) the electric organ, and (c) gonads (light microscopy, (b) the electric organ, and (c) gonads (light microscopy) using established tissue staining techniques; (3) androgen effects on cell birth and/or growth of anal-fin bone, cartilage, muscle and motoneurons using immunocytochemical (ICC) BrdU birthdating techniques; (4) HRP tract tracing to determine the control of the anal-fin reflex (spinal cord motoneuronal pools); (5) using ICC methodology to estimate the density of androgen receptors in those targets implicated in the control of anal-fin reflex, and (6) determine the amount of circulating plasma androgen levels (using commercially available test kits). The following questions will be addressed: (1) How does the increase in bone and muscle size come about (cell birth and/or growth); (2) what is the role of the fish's androgens in mediating cellular changes affecting muscle, bone, and cartilage, as well as motoneuronal pools; (3) what is the distribution of androgen receptors in these structures; (4) why, responding to hormone treatment and withdrawal, is gonadal steroid sensitivity flexible (plastic) in some targets and rather rigid (fixed) in others, and finally, (5) how do the answers to these questions contribute to a comprehensive understanding of the expression of courtship display structures and behavior.